Abstract

Parasites and pathogens are usually harmful, and thus hosts have developed numerous pre‐contact measures to avoid infection
in the first place. These avoidance measures range from camouflage to a variety of behavioural measures such as movement and
fly‐repelling behaviour, habitat choice and migration, selective foraging, group forming and mate choice. In case avoidance
fails, a second line of post‐contact defences (e.g. immune system) comes into place which is treated elsewhere and thus only
briefly discussed.

Keywords: parasitism; avoidance

Figure 1.

Examples of pre‐contact avoidance measures as a first line of defence against different types of parasites and pathogens in
birds. These are mainly behavioural measures to avoid contact with parasites and pathogens in the first place. Once a host
has come into contact with parasites and pathogens, there is a second line of defence in the form of post‐contact defensive
measures which can be either behavioural, physiological or immunological. Here we focus on pre‐contact avoidance measures.
Figure based on Hart . Copyright of David Thieltges.

Figure 2.

Mechanisms of parasite and pathogen avoidance or defence are costly and thus reduce the fitness of resistant hosts compared
to susceptible hosts in the absence of parasites. However, in the presence of parasites, the fitness of resistant hosts is
higher than that of susceptible hosts with the latter suffering more from the costs of an infection. Pre‐contact avoidance
and post‐contact defensive measures can only evolve if the cost of infection exceeds the cost of avoidance and defensive measures.
Copyright of David Thieltges.

Figure 3.

Selective prey selection can help to avoid acquiring parasites and pathogens. Oystercatchers take smaller cockle prey individuals
with suboptimal energy intake. Smaller cockles harbour fewer infective stages of trematode parasites (which utilize the birds
as final hosts) compared to larger cockles. By preferring smaller cockles the birds thus reduce their parasite intake and
avoid acquiring high loads of parasites. Total avoidance of parasites is not possible in this case because all cockle sizes
are infected to some extent. Schematic graph based on data taken from Norris . Copyright of David Thieltges.

Figure 4.

Forming groups can help to avoid parasites and pathogens. In birds and mammals, individual hosts suffer from reduced fly bites
with increasing group size (dilution) as long as the larger group does not attract more flies due to a higher visibility (encounter).
Positioning themselves at the centre of a large group can also help individual hosts to avoid fly bites and potential subsequent
infections by the selfish–herd effect. Figure based on Hart , by permission of Oxford University Press (www.oup.com).

Figure 5.

Example of post‐contact defensive measures against parasites and pathogens. Invertebrate immune systems can distinguish between
self and nonself and eliminate parasites or pathogens for example by encapsulation and melanization. The picture shows an
amphipod host in which invading trematode parasites (arrows) have been killed by encapsulation. Copyright of Robert Poulin.

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